Basic idea of the present paper. Left panel: bolometric X-ray source luminosity L_eqpair (in units of 10³⁸ erg s⁻¹) of Fig. 2 is shown versus the EUV irradiation from the X-ray source (EF_E in units of 2 × 10³⁶ erg s⁻¹ at 100 eV; solid line). Around it, L_rad is variable and two positions are shown (dashed lines). When L_rad − L_eqpair = ϵ is positive, the evolution goes upward, “UP” (1-2-3, positive feedback), when negative it goes downward, “DOWN” (4-5-6, negative feedback). Right panel: zoomed version of the panel on the left.